In an imaging device such as a digital still camera, an image of subject is captured by using a solid-state image sensor such as a CCD (Charge Coupled Device) and a CMOS image sensor (Complementary Metal Oxide Semiconductor Image Sensor). Each of these solid-state image sensors has a spectral sensitivity of from visible region to a near-infrared-wavelength region in the vicinity of 1100 nm, and so it is not possible to obtain a good color reproducibility as it is. Thus, the spectral sensitivity is corrected to correspond to a normal luminosity factor of human being by using a filter which shields light in the infrared-wavelength region. Specifically, the filter that shields light in the infrared-wavelength region is provided in an optical path from an imaging lens to the solid-state image sensor. The filter used for such purpose is required to have a high transmittance of light in the visible wavelength region, and a dielectric multilayer in which each of a plurality of high-refractive index layers and each of a plurality of low-refractive index layers are alternately stacked is employed, for example.
However, the filter having the dielectric multilayer has an incident angle dependence of light. The incident angle dependence shifts a cut-off wavelength when the filter is applied to the imaging device and the incident angle of light through the imaging lens with the filter varies. Thus, a color tone at a center portion of a shot image and a color tone at a peripheral portion of the shot image may be changed. Hereinafter, the incident angle dependence of light is simply referred to as the incident angle dependence.
A method of reducing the incident angle dependence is known. The method uses a first dielectric multilayer and a second dielectric multilayer. A width of reflecting band of the first dielectric multilayer is set to be narrower than a width of reflecting band of the second dielectric multilayer. The position of a short-wavelength-side edge of the second dielectric multilayer is set to be at a position between a short-wavelength-side edge and a long-wavelength-side edge of the reflecting band of the first dielectric multilayer. In particular, an average refractive index of the entire first dielectric multilayer is set to be higher than an average refractive index of the entire second dielectric multilayer. In the method as described above, the incident angle dependence is reduced by the first dielectric multilayer having the high average refractive index, and a wide reflecting band is secured by using the first dielectric multilayer together with the second dielectric multilayer.
Further, a dichroic mirror is known. The dichroic mirror has a first selective transmission multilayer formed of a high-refractive index layer and a medium-refractive index layer, and a second selective transmission multilayer formed of a high-refractive index layer and a low-refractive index layer with a large refractive index ratio. In the dichroic mirror, the incident angle dependence is reduced by the first selective transmission multilayer formed of the high-refractive index layer and the medium-refractive index layer.